Clinical experience has documented that mental disorders commonly occur among individuals with intellectual disabilities. However, accurate estimates of the epidemiological parameters of mental illness among persons with intellectual disabilities are not available. Indeed, prevalence estimates of psychopathology for this population vary widely in both clinical and community settings. Reasons for the imprecise epidemiological data include the lack of valid approaches to identifying mental disorders in the context of intellectual disabilities. Hence, there is a need for validated instruments and procedures to address the potential biases inherent in the assessment of individuals with intellectual disabilities. The proposed research will aim to address this urgent need in the following two phases. In the first phase, we will conduct formative development activities to adapt the Diagnostic Interview Schedule for Children (DISC) for assessing DSM-5 mental disorders among adolescents and young adults with intellectual disabilities (ages 14 to 21 years). An iterative development and testing process, including expert reviews and pilot testing, will be used to examine (a) lexical adaptations to interview symptom probes, (b) response option formats to address the potential of acquiescence bias and limitations in understanding response options, and (c) time frame and timeline follow-back procedures for assessing past symptoms and disorder episodes in a population with difficulty in articulating the passage of time. Interview formats and assessment protocols will be developed for direct reports by youth and informant reports by caregivers. In the second phase, a field test will be conducted to examine the reliability and validity of the modified DISC interview formats with a diverse sample of 400 adolescents and young adults and their caregivers recruited from both clinical and community settings. The proposed project will likely improve our knowledge regarding valid approaches to assessing mental disorders among adolescents and young adults with intellectual disabilities. If the aims of the project are achieved, the validated assessment approach will contribute to subsequent epidemiological research and clinical practices addressing the mental health needs of individuals with intellectual disabilities.